1 //! Extensions to the parsing API with niche applicability. 2 3 use super::*; 4 5 /// Extensions to the `ParseStream` API to support speculative parsing. 6 pub trait Speculative { 7 /// Advance this parse stream to the position of a forked parse stream. 8 /// 9 /// This is the opposite operation to [`ParseStream::fork`]. You can fork a 10 /// parse stream, perform some speculative parsing, then join the original 11 /// stream to the fork to "commit" the parsing from the fork to the main 12 /// stream. 13 /// 14 /// If you can avoid doing this, you should, as it limits the ability to 15 /// generate useful errors. That said, it is often the only way to parse 16 /// syntax of the form `A* B*` for arbitrary syntax `A` and `B`. The problem 17 /// is that when the fork fails to parse an `A`, it's impossible to tell 18 /// whether that was because of a syntax error and the user meant to provide 19 /// an `A`, or that the `A`s are finished and its time to start parsing 20 /// `B`s. Use with care. 21 /// 22 /// Also note that if `A` is a subset of `B`, `A* B*` can be parsed by 23 /// parsing `B*` and removing the leading members of `A` from the 24 /// repetition, bypassing the need to involve the downsides associated with 25 /// speculative parsing. 26 /// 27 /// [`ParseStream::fork`]: ParseBuffer::fork 28 /// 29 /// # Example 30 /// 31 /// There has been chatter about the possibility of making the colons in the 32 /// turbofish syntax like `path::to::<T>` no longer required by accepting 33 /// `path::to<T>` in expression position. Specifically, according to [RFC 34 /// 2544], [`PathSegment`] parsing should always try to consume a following 35 /// `<` token as the start of generic arguments, and reset to the `<` if 36 /// that fails (e.g. the token is acting as a less-than operator). 37 /// 38 /// This is the exact kind of parsing behavior which requires the "fork, 39 /// try, commit" behavior that [`ParseStream::fork`] discourages. With 40 /// `advance_to`, we can avoid having to parse the speculatively parsed 41 /// content a second time. 42 /// 43 /// This change in behavior can be implemented in syn by replacing just the 44 /// `Parse` implementation for `PathSegment`: 45 /// 46 /// ```edition2018 47 /// # use syn::ext::IdentExt; 48 /// use syn::parse::discouraged::Speculative; 49 /// # use syn::parse::{Parse, ParseStream}; 50 /// # use syn::{Ident, PathArguments, Result, Token}; 51 /// 52 /// pub struct PathSegment { 53 /// pub ident: Ident, 54 /// pub arguments: PathArguments, 55 /// } 56 /// # 57 /// # impl<T> From<T> for PathSegment 58 /// # where 59 /// # T: Into<Ident>, 60 /// # { 61 /// # fn from(ident: T) -> Self { 62 /// # PathSegment { 63 /// # ident: ident.into(), 64 /// # arguments: PathArguments::None, 65 /// # } 66 /// # } 67 /// # } 68 /// 69 /// impl Parse for PathSegment { 70 /// fn parse(input: ParseStream) -> Result<Self> { 71 /// if input.peek(Token![super]) 72 /// || input.peek(Token![self]) 73 /// || input.peek(Token![Self]) 74 /// || input.peek(Token![crate]) 75 /// || input.peek(Token![extern]) 76 /// { 77 /// let ident = input.call(Ident::parse_any)?; 78 /// return Ok(PathSegment::from(ident)); 79 /// } 80 /// 81 /// let ident = input.parse()?; 82 /// if input.peek(Token![::]) && input.peek3(Token![<]) { 83 /// return Ok(PathSegment { 84 /// ident: ident, 85 /// arguments: PathArguments::AngleBracketed(input.parse()?), 86 /// }); 87 /// } 88 /// if input.peek(Token![<]) && !input.peek(Token![<=]) { 89 /// let fork = input.fork(); 90 /// if let Ok(arguments) = fork.parse() { 91 /// input.advance_to(&fork); 92 /// return Ok(PathSegment { 93 /// ident: ident, 94 /// arguments: PathArguments::AngleBracketed(arguments), 95 /// }); 96 /// } 97 /// } 98 /// Ok(PathSegment::from(ident)) 99 /// } 100 /// } 101 /// 102 /// # syn::parse_str::<PathSegment>("a<b,c>").unwrap(); 103 /// ``` 104 /// 105 /// # Drawbacks 106 /// 107 /// The main drawback of this style of speculative parsing is in error 108 /// presentation. Even if the lookahead is the "correct" parse, the error 109 /// that is shown is that of the "fallback" parse. To use the same example 110 /// as the turbofish above, take the following unfinished "turbofish": 111 /// 112 /// ```text 113 /// let _ = f<&'a fn(), for<'a> serde::>(); 114 /// ``` 115 /// 116 /// If this is parsed as generic arguments, we can provide the error message 117 /// 118 /// ```text 119 /// error: expected identifier 120 /// --> src.rs:L:C 121 /// | 122 /// L | let _ = f<&'a fn(), for<'a> serde::>(); 123 /// | ^ 124 /// ``` 125 /// 126 /// but if parsed using the above speculative parsing, it falls back to 127 /// assuming that the `<` is a less-than when it fails to parse the generic 128 /// arguments, and tries to interpret the `&'a` as the start of a labelled 129 /// loop, resulting in the much less helpful error 130 /// 131 /// ```text 132 /// error: expected `:` 133 /// --> src.rs:L:C 134 /// | 135 /// L | let _ = f<&'a fn(), for<'a> serde::>(); 136 /// | ^^ 137 /// ``` 138 /// 139 /// This can be mitigated with various heuristics (two examples: show both 140 /// forks' parse errors, or show the one that consumed more tokens), but 141 /// when you can control the grammar, sticking to something that can be 142 /// parsed LL(3) and without the LL(*) speculative parsing this makes 143 /// possible, displaying reasonable errors becomes much more simple. 144 /// 145 /// [RFC 2544]: https://github.com/rust-lang/rfcs/pull/2544 146 /// [`PathSegment`]: crate::PathSegment 147 /// 148 /// # Performance 149 /// 150 /// This method performs a cheap fixed amount of work that does not depend 151 /// on how far apart the two streams are positioned. 152 /// 153 /// # Panics 154 /// 155 /// The forked stream in the argument of `advance_to` must have been 156 /// obtained by forking `self`. Attempting to advance to any other stream 157 /// will cause a panic. advance_to(&self, fork: &Self)158 fn advance_to(&self, fork: &Self); 159 } 160 161 impl<'a> Speculative for ParseBuffer<'a> { advance_to(&self, fork: &Self)162 fn advance_to(&self, fork: &Self) { 163 if !private::same_scope(self.cursor(), fork.cursor()) { 164 panic!("Fork was not derived from the advancing parse stream"); 165 } 166 167 // See comment on `cell` in the struct definition. 168 self.cell 169 .set(unsafe { mem::transmute::<Cursor, Cursor<'static>>(fork.cursor()) }) 170 } 171 } 172